I agree that CE is on it's way out.
This is because the syllabus itself is very old fashioned in terms of being so content heavy and involving lots of rote learning, in a time where teaching is less based on this now. It is also because both schools and parents want earlier certainty about school places.
Even schools which still nominally use CE tend to pre-test in Y6 now and offer places then - these tests are often ISEB multiple choice which are a version of an IQ test and a bit like the multiple choice exams sat for 11+ but can also involve reading comprehension, creatives writing and maths. Schools either offer purely based on these tests in Yr6 or offer based on them and add a CE requirement which candidates never fail to achieve. CE essentially then becomes a way of keeping yr 7 and 8s motivated after they have received their offers. Some schools also offer a purely Yr8 test (again an internal test) and CE requirement or offer both a Yr6 pre test and Yr8 test option, to help the Prep schools cope with change......often boys schools in particular historically thought some boys in Yr6 weren't mature enough to do their best in tests in Yr6 and would do better if tested in Yr8 (at least that's what was said....but perhaps it was partly to ensure those boys remained until yr8)
Either way fewer schools do CE each year and fewer Senior schools require it. Prep schools can only decide to abandon it if the Seniors they feed are happy not to have it, or the Seniors themselves drive the move away from it. St Pauls and Westminster both announced they would no longer use CE so there is momentum in this direction from the big names too.
It's a hard one for Prep schools that go to 13. Teaching CE was a bit of a burden, but was a way they made themselves distinctive. Now they don't do that, and especially if kids have their offers mid-Yr6 it could be quite hard to keep them focused.
I think it's all part of the trend to move away from 13+ entry too. Although a number of boarding schools only offer 13+ entry so take in from 13+ Preps, more and more senior schools take all or an increasing bulk of their entry at 11+. It secures 7 years of fees for them and greater certainty of who is coming. Prep schools which feed both 11+ and 13+ schools can find their numbers in Yr7 and 8 dwindle, as parents often opt for the cheaper and highly successful day schools which are predominantly taking in at 11. Many parents are wary of waiting for true 13+ entry in Yr8 and knowing that most schools now test in Yr6, decide the children might as well move in Yr7 not Yr9. So whilst I'm sure some 13+ Prep schools will keep going to feed the smallish number of 13+ only entry schools, the trend will be that more Prep schools abandon their Yr7 and 8 as they become less viable due to smaller numbers, especially those that predominantly send their children to day schools.
Interesting changes, driven I think by the fact boarding fees become increasingly out of reach, schools compete with each other to attract the day market in at Yr7 for more years of fees, and parents wanting certainty sooner,in what often feels a highly competitive market.